The objective of the research is the evaluation of the dynamic properties of the membranes of cardiac cells and autonomic nerve cells during development of the avian heart. Intracellular microelectrode recording techniques will be used to obtain quantitative information about the electrical activity of individual cardiac cells during growth and development of the chick embryo. Examination of the changes in intracellular membrane potentials during maturation will assist in understanding the regulation of ion transport and the maintenance of cellular homeostasis by cardiac membranes. This follows from the fact that cellular membrane potentials are resultant of the transmembrane distribution of ions and membrane permeability to ions. Pharmacologic agents, known to modify ionic conductance and membrane potential in hearts from adult animals, will be used as probes of embryo heart membrane electrical activity. In this manner, alterations in reactivity to drugs will serve as indices of ontogenetic changes that occur in the drug receptor and in the ionic conductance mechanism(s) regulated by the drug-receptor interaction. The role of the autonomic nervous system in modulating intrinsic cardiac activity and membrane reactivity to applied drugs will be examined by the use of aneural and innervated hearts. The ultrastructural features of synaptogenesis and the sensitivity of autonomic nerves to excitatory and inhibitory drugs will be studied in order to ascertain the neurosecretory properties of developing nerves. Summarily, this study can provide a better understanding of the pharmacology and physiology of the heart during a critical biologic period.